The production of light gauge steel for end uses ranging from architectural construction materials to automobiles is well known. A rolling mill line produces continuous sheets of steel in the required thickness and width. The steel sheets may be coated with a thin layer of zinc metal via a galvanizing process. Eventually, mill oil is applied to the uncoated or galvanized steel sheets, and the steel is either stored or shipped in a coil to a customer for further processing.
Typically, the customer is an automobile manufacturer who will take the coiled metal sheet and pass it through a lubricating station and then to a forming operation where the metal sheet is cut and formed into a part such as a roof, fender, door, etc. The parts are then welded together to form an automobile body. Next, the automobile body is cleaned, treated with a zinc phosphating solution to enhance corrosion protection, and rinsed with deionized water. The treated automobile body is then passed through an electrodeposition bath where a corrosion resistant primer is applied.
The automobile manufacturers would like to streamline their operations and have some of the operations described above done outside the automobile assembly plant, for example at a steel mill or a custom coater. One major problem with moving certain operations to a steel mill or a custom coater is that any coating applied outside the automobile assembly plant must be able to accept a weld. At some point in time, the various metal parts will be welded together in the automobile assembly plant to form the automobile body. Consequently, automobile manufacturers have a strong demand for a weldable, corrosion resistant coating composition that can be applied at a steel mill or at a custom coating facility.
Such a weldable, corrosion resistant coating composition could be applied at a custom coater, known as a coil coater, who would ship the coated metal sheet to the automobile manufacturer. As described above, the automobile manufacturer would then form the metal sheet into parts and weld the parts together. However, the metal pretreatment operation and perhaps the electrodeposition process could be avoided since the metal received by the automobile manufacturer would already contain a corrosion resistant coating.
Similar to the above, a weldable, corrosion resistant coating composition could also be applied at a steel mill. Application at the steel mill enables the automobile manufacturer to receive corrosion resistant metal directly without the expense associated with shipping the metal to a coil coater and from the coil coater to the automobile manufacturer.
The present invention provides a weldable, curable coating composition that provides corrosion protection and can be applied by a coil coater or at a steel mill, can be cured at low temperature and provides good adhesion and good corrosion protection without prior metal pretreatment.